In general, carbon dioxide (CO2) accounts for about 60% of the overall green house gases, and is a main factor of the global warming phenomenon. Carbon dioxide has been subjected to efforts and regulations to reduce its emission early on, on the basis of the United Nations Framework Convention on Climate Change (UNFCCC).
However, consumption of fuel for generating energy continues to increase, and thus it is difficult to reduce the emission of carbon dioxide itself.
In this regard, due to the recent introduction of carbon dioxide emission rights, each enterprise must reduce its emission of carbon dioxide to an allocated target value. If not, the carbon dioxide emission rights must be additionally purchased for the shortage, and thus the enterprises lose competitive power due to an increase in cost. In contrast, the enterprises that reduce their emission of carbon dioxide exceeding the allocated target value, can sell the carbon dioxide emission rights corresponding to the exceeded value, and thus create additional profits along with an image of an eco-friendly enterprise.
Thus, each enterprise has made a great deal of efforts to develop carbon dioxide reduction technology.
Particularly, in high energy consumption systems and processes such as thermoelectric power plants, reaction processes of producing biogas, biogas combustion systems, steel mills, cement factories, and so on, a large amount of carbon dioxide is discharged by the following mechanisms.
1. Thermoelectric Power PlantsC+O2→CO2↑(generation of a large amount of carbon dioxide)
2. Reaction Processes of Generating and Producing BiogasOrganic material+Digestive process→CH4+CO2 (biogas)Reclaimed waste+Microorganism degradation→CH4+CO2 (biogas)
3. Biogas Combustion Systems (all Heat Engines Such as Thermoelectric Power Plants Using Liquefied Natural Gas (LNG) as Fuel)CH4+2O2→2H2O↑+CO2↑(generation of a large amount of carbon dioxide)
4. Main Reaction of Refining Iron Ore at Steel MillsMagnetite: Fe3O4+2C→3Fe+2CO2↑(generation of a large amount of carbon dioxide)Hematite: Fe2O3+3Co→2Fe+3CO2↑(generation of a large amount of carbon dioxide)
5. Cement FactoriesCaCO3→CaO+CO2↑(generation of a large amount of carbon dioxide)
First, as coal fuel is burnt at the thermoelectric power plants, a large amount of carbon dioxide is generated by the mechanism (No. 1) above, and is contained in exhaust gases. For this reason, carbon dioxide generated at the thermoelectric power plants must be positively reduced.
Furthermore, inorganic residues, coal ash (i.e. ash), left after coal is burnt as fuel in the furnace of a coal thermoelectric power plant are generated as by-products. The coal ash is divided into bottom ash, which falls to and is deposited at a lower portion of the furnace, and fly ash, which is discharged through a flue at an upper portion of the furnace along with exhaust gases.
The fly ash is discharged through the flue in scattered particles of a fine size along with the exhaust gases, and thus is collected by a dust collector installed on the flue.
The bottom ash is formed so as to be changed into a somewhat large particle in the furnace by a sintering process, falls to and is deposited at a lower portion of the furnace, and is typically collected and pulverized into a size of about 1 mm to about 10 mm by a mill.
The coal ash was mainly buried in landfills additionally prepared at the power plant in earlier times. Recently though, due to a sharp increase in the price of the land, it has been difficult to secure landfills, and thus to dispose of the coal ash in the landfills.
In addition, the coal ash has been positively recycled recently. For example, fly ash has been mainly recycled for cement additives, fillers, soil conditioners, light-weight aggregates, and so on, and bottom ash has been mainly recycled for road bed fillers, base materials for pavement, aggregates for concrete mixture, and so on.
However, the recycled percentage has been low up to now, and a considerable part of coal ash has been simply buried. Thus, it is necessary to further extend the field capable of recycling the coal ash.
Meanwhile, the coal ash has a difference in their components depending on a kind of burned coal, a type of furnace, and so on, and basically contain calcium oxide (CaO) of about 2% to about 45%.
Thus, when the coal ash is buried or recycled, the contained CaO reacts with water, thereby producing calcium hydroxide (Ca(OH)2) showing strong alkalinity greater than pH 12. As such, when recycled as a component for a structure, the coal ash cause damage to the structure due to hydration expansion, and are subjected to a partially bulging phenomenon. In contrast, when recycled as a component for reclamation or construction, the coal ash come into contact with underground water or rainwater, thereby producing strong alkaline leachate. This strong alkaline leachate is discharged to rivers or seas, thereby posing a risk of causing severe water pollution.
Accordingly, the coal ash is previously neutralized to prevent environmental pollution or structural damage from being caused when buried or recycled, and their recyclability needs to be expanded to other uses.
However, in carrying out neutralization and harmless treatment on the coal ash, excessive expenses or other environmental pollutions must not be incurred.
In other words, a satisfactory processing method in the economical and eco-friendly aspect must be adopted.
Next, biogas is burnt in the furnace for conversion into other energy such as electricity. Methane gas contained in the biogas reacts with oxygen. Thereby, a large amount of carbon dioxide is generated by the mechanism (No. 3) above, and is contained in exhaust gases. For this reason, carbon dioxide must be positively reduced in the biogas combustion systems.
One of the most anticipated alternative fuels to fossil fuel is biogas, which is expected to be able to be sufficiently put to practical use along with natural gas in the near future.
The biogas is automatically obtained by fermenting organic waste having a high content of biomass (organic material) such as livestock excrement, food waste, or sludge of a sewage disposal plant. The organic waste may be sufficient as a raw material, because it is continuously generated by human activities and various industrial activities.
In detail, when the organic waste containing a large quantity of organic material is digested in an anaerobic state where no oxygen exists, the biogas is produced by degradation of the organic material on the basis of the mechanism (No. 2) above, and is generated at the sewage disposal plant by the degradation of microorganisms on the basis of the mechanism (No. 2) above. The biogas is mainly made up of methane (CH4) of about 60% to about 70% and carbon dioxide (CO2) of about 30% to about 40%. That is, a large quantity of carbon dioxide is discharged when the biogas is produced and generated.
Here, for the anaerobic digestion, an anaerobic filter method in which carriers are installed in an anaerobic digestion tank using the adhesion of anaerobic microorganisms, or an upflow anaerobic sludge blanket (UASB) method using granulation based on self immobilization of microorganism is typically applied.
Since the biogas contains a large quantity of methane gas of about 60% to about 70%, it is typically burnt, thereby obtaining electric energy and heat energy.
However, when the biogas is burnt, a large quantity of carbon dioxide of about 30% to about 40% inhibits oxidation. As such, an amount of heat generated by combustion is small, and the resulting rate of conversion into electric energy and heat energy is low.
Thus, a great deal of effort has been made to solve this problem.
For example, there is a method of separating carbon dioxide contained in the biogas bit by bit, discharging it into the air, increasing the contained methane gas to a content of about 75% to about 80%, and reducing the contained carbon dioxide to a content of about 20%.
However, this method has a problem in that it cannot remove carbon dioxide exceeding that content.
Then, a large quantity of carbon dioxide (CO2) is generated and discharged by the mechanisms (Nos. 3 and 4) accompanied with the combustion of fuel along with industrial waste such as slag and waste acid in a process of producing steel at the steel mill, a process of producing cement at the cement factory, or the like. For this reason, carbon dioxide must be positively reduced at the steel mills and the cement factories.
For recycling or harmless treatment of the industrial waste such as slag and waste acid, high expenses are additionally required, causing a burden on cost, and environmental pollution is additionally caused in the process.
However, research and development for reducing environmental pollutants such as carbon dioxide in the various fields as mentioned above have not yet produced satisfactory remarkable results in the aspects of efficiency, economical efficiency, industrial application value, and on-the-spot applicability.